1. Field of the Invention
This invention relates to a novel bilayered phosphoric acid anodized aluminum support for use in a lithographic printing plate, to a method for the preparation of such support, and to a lithographic printing plate comprising the support which exhibits improved resistance to staining in non-image areas and good adhesion.
2. Description of the Prior Art
U.S. Pat. No. 4,647,346 describes a superior lithographic printing plate having improved abrasion resistance comprising a phosphoric acid anodized aluminum support surface sufficiently porous to achieve adequate adhesion. This plate has achieved widespread commercial acceptance. However, such plate has demonstrated on occasion, and for reasons that are not completely understood, a tendency to stain in non-image areas. Poor resistance to staining manifests itself during the printing operation as an increase in density in non-image areas. This is believed to result, at least in part, from ink retention in the non-image areas. Such stain can cause press operators to make unnecessary adjustments during press runs and, if severe, can cause ink to be transferred to the non-image areas of the print.
Attempts have been made to increase the resistance to staining of the plate described in the above-noted patent. However, such attempts, prior to this invention, have met with only limited success in that improved stain resistance has been achieved only at the expense of an unacceptable decrease in surface adhesion.
Thus, there is a need for a lithographic printing plate having the advantages of the plate described in U.S. Pat. No. 4,647,346 yet having improved resistance to staining while retaining good adhesion.